1. Field
The present disclosure generally relates to systems and methods of measuring of acceleration due to gravity and, more particularly, gravimeters using signal processing to provide synthetic vibration isolation.
2. Description of the Related Art
Gravimetry is a branch of science concerned with measurement of gravitational fields. Precision measurement of the earth's gravitational field is important both for fundamental scientific research and for the exploration of oil and mineral resources. Gravity is usually measured in units of acceleration. One commonly used unit of acceleration is the “g”, where 1 g is the standard value of earth's gravitational acceleration at sea level and, more precisely, has a value of 9.80665 meters per second squared (9.80665 m/s2) as declared at the 3rd Conférence Générale des Poids et Mesures (CGPM) in 1901. The derived unit of a “micro-g”, i.e. a millionth of a g or 9.80665 μm/s2, is sometimes used when dealing with small variations in gravity. Another unit of acceleration used extensively in the science of gravimetry is the “gal”, named in honor of the Italian physicist Galileo Galilei. The gal is defined as 1 centimeter per second squared (1 cm/s2).
Precision measurements of gravity are also used to map the shape of the earth, which is the science of geodesy, as the value of the acceleration due to gravity changes with distance from the center of the earth. Post-glacial rebound, the visco-elastic response of the Earth to the melting of the large ice sheets of the ice ages, currently is expressed in vertical crustal motions on the order of one millimeter per year over the entire Earth. The crustal rebound exceeds 1 centimeter per year in regions of North America and Europe, where the major ice sheets existed 20,000 years ago. The change in gravity caused by a change in the distance of a point on the surface of the Earth from the center of the mass is approximately 3 microgal per centimeter. The ability to measure a change in gravity of 1 microgal therefore corresponds to a resolution of 3 millimeters in vertical crustal motion, assuming that other causes of changes in gravity can be properly accounted for. As the gravitational acceleration on the Earth's surface is on the order of 980 gal, measuring gravity to an accuracy of 1 microgal requires a resolution of approximately 1 part per billion (10−9).
People have been building instruments to measure the acceleration due to gravity, called gravimeters, since 1680. The methods of measuring the acceleration due to gravity have evolved from pendulums that could measure gravity to 1 part in 10,000 (10−4) to today's instruments that drop an object in a vacuum and measure its position as it free falls over time. Some instruments drop a mass having a retroreflector, an optical device that reflects any incident beam of light directly back at the source, and repeatedly measure the displacement of the mass as it falls using optical interferometry. An ideal curve is then fit to these displacement measurements to calculate the acceleration of the falling mass. This process of dropping a mass may be repeated hundreds of times and the results combined to further reduce the uncertainty of the composite value of the acceleration.
Conventional free-fall gravimeters observe a mass free-falling for a distance on the order of tens of centimeters in a vacuum chamber. The position of the falling mass is typically measured using interferometric techniques, bouncing a laser beam off a retroreflector on the falling mass and off a reference retroreflector that is mounted on an active vibration isolation system that has a period of approximately 60 seconds. While the accuracy of a conventional free-fall gravimeter is on the order of 10−9, gravimeters of this type may weigh up to 350 kg and have a height of up to 1.5 meters tall. Much of this weight and size is attributable to the complexity of providing a reference retroreflector that approximates a stationary reference in inertial space. This size and weight present limits to the usability of conventional gravimeters.